There is a conventionally known outboard engine system including a catalytic converter incorporated in an exhaust system for an engine, for purifying an exhaust gas (for example, see Japanese Utility Model Publication No.59-18092).
It should be noted here that the catalytic converter exhibits the highest purifying function in an extremely narrow range A of A/F ratio of an exhaust gas around a theoretic A/F ratio (14.6), as shown in FIG. 17. Therefore, when a catalytic converter is used for purifying an exhaust gas from an engine, it is a conventional common practice to detect an A/F ratio of an exhaust gas by an O2 sensor varying an output largely at about a theoretic A/F ratio, and to control the A/F ratio of an air-fuel mixture drawn into the engine, so that the O2 sensor produces an output corresponding to the theoretic A/F ratio, whereby the exhaust gas is always purified with a good efficiency by the catalytic converter.
The O2 sensor varies the output largely relative to a variation in A/F ratio in the range A, but a variation in output relative to a variation in A/F ratio is little produced in a range other than the range A. Therefore, the use of the O2 sensor is advantageous for controlling the A/F ratio of the exhaust gas to the theoretic A/F ratio with a good efficiency, but when the tolerance of the purifying rate of the catalytic converter is set relatively widely with the engine output taken into consideration to a certain extent, it is extremely difficult to carry out the control of the A/F ratio with the O2 sensor.